arXiv subjects

Charles A. Collett

Publications and source records attributed to Charles A. Collett.

8 recordsLinked to original sources

arXiv2025

Enhancing Coherence with a Clock Transition and Dynamical Decoupling in the Cr$_7$Mn Molecular Nanomagnet

Molecular magnets are attractive as spin qubits due to their chemical tunability, addressability through electron-spin resonance techniques, and long coherence times. Clock transitions (CTs), for which the system is immune to the effect of magnetic-field fluctuations to first order, provide a method to enhance the coherence time $T_2$, and to reveal mechanisms of decoherence that are not due to such fluctuations. Here we investigate two variants of Cr$_7$Mn, a spin-1 molecular nanomagnet, at fields near a zero-field CT. We find that at temperatures $\le$2 K, $T_2\sim1$ $μ$s at the CT using a Hahn-echo pulse sequence. Away from the CT, electron-spin-echo envelope modulation (ESEEM) oscillations due to coupling to nuclear spins are observed and have a $T_2$ as high as $1.35$ $μ$s, indicating a distinct mechanism of coherence preservation. Dynamical decoupling with the CPMG pulse sequence yields $T_2\sim\!2.8$ $μ$s at the CT and up to $\sim\!3.6$ $μ$s in the ESEEM regime along with a demodulation of the oscillatory behavior. The experimental values of $T_2$ are largely independent of the degree of dilution of the molecules in solvent or whether the solvent is deuterated, indicating that much of the decoherence and ESEEM arises from sources within the molecules themselves. To account for decoherence, we develop a model that includes not only field fluctuations but also fluctuations in the CT transition frequency itself. Our results can be well explained by treating the environment as a combination of noise at the nuclear Larmor precession frequency and $1/f$ noise in the transverse anisotropy parameter $E$. Such information about the microscopic origins of decoherence can aid the rational design of molecular-based spin qubits.

cond-mat.mes-hall↗

arXiv2024

An Inexpensive, Configurable Two-Tone Electron Spin Resonance Spectrometer

Electron spin resonance (ESR) is a powerful tool for characterizing and manipulating spin systems, but commercial ESR spectrometers can be expensive and designed to work in narrow frequency bands. This work presents an inexpensive spectrometer that, when coupled with easy-to-design resonators, enables ESR over a broad frequency range, including at frequencies outside the standard bands. It can operate at either a single frequency or at two frequencies simultaneously. The spectrometer is built from off-the-shelf parts and controlled by a field programmable gate array (FPGA), and new capabilities can be easily added by reconfiguring the FPGA and adding or swapping components. We demonstrate the capabilities of the spectrometer using the molecular nanomagnet Cr$_7$Mn.

physics.ins-det↗

arXiv2023

The loop-zag resonator: A loop-gap resonator design for improved sensitivity in electron-spin resonance experiments

We present a novel design of loop-gap resonator, the loop-zag resonator, for sub-X-band electron-spin resonance spectroscopy. The loop-zag design can achieve improved coupling to small-sample spin systems through the improvement of sample filling factor and RF $B_1$ field. By introducing ``zags'' to the resonator's gap path, the capacitance is increased, accommodating a smaller loop size and thereby a larger filling factor to maintain the requisite resonant frequency. We present experimental spectra on five different resonators, each with approximately the same resonant frequency of $\sim2.9$~GHz, showing that an increase in the number of zags and reduction in loop size gives rise to higher sensitivity. Finite-element simulations of these resonators provide estimates of the improved filling factors obtained through the addition of zags. The frequency range over which this loop-zag design is practical enables a breadth of future applications in microwave engineering, including ESR and ESR-like quantum information microwave techniques.

cond-mat.mes-hall↗

arXiv2020

Direct Spectroscopic Observation of Berry Phase Interference in the Ni$_4$ Single-Molecule Magnet

Berry phase effects in spin systems lead to the suppression of tunneling effects when different tunneling paths interfere destructively. Such effects have been seen in several single-molecule magnets (SMMs) through measurements of magnetization dynamics, where the experimental signal may arise from the contributions of numerous energy levels. Here we present experimental measurements of Berry phase interference effects that are determined through electron-spin resonance on a four-fold symmetric SMM. Specifically, we measure transitions between tunnel-split excited states in the Ni$_4$ SMM in the presence of a transverse field in the hard plane of the crystalline sample. By using a home-built rotation apparatus, the direction of the sample can be changed \textit{in situ} so that that the field direction can be swept through the entire hard plane of the sample. When the field is in certain directions in the plane, we observe a splitting of the transition, a hallmark of Berry phase interference. The experimental results are well reproduced by theoretical predictions, and fitting of the data provides information about the effects of dipolar interactions and sample misalignment.

cond-mat.mes-hall↗

arXiv2020

Constructing Clock-Transition-Based Two-Qubit Gates from Dimers of Molecular Nanomagnets

A good qubit must have a coherence time long enough for gate operations to be performed. Avoided level crossings allow for clock transitions in which coherence is enhanced by the insensitivity of the transition to fluctuations in external fields. Because of this insensitivity, it is not obvious how to effectively couple qubits together while retaining clock-transition behavior. Here we present a scheme for using a heterodimer of two coupled molecular nanomagnets, each with a clock transition at zero magnetic field, in which all of the gate operations needed to implement one- and two-qubit gates can be implemented with pulsed radio-frequency radiation. We show that given realistic coupling strengths between the nanomagnets in the dimer, good gate fidelities ($\sim$99.4\%) can be achieved. We identify the primary sources of error in implementing gates and discuss how these may be mitigated, and investigate the range of coherence times necessary for such a system to be a viable platform for implementing quantum computing protocols.

quant-ph↗

arXiv2018

A Clock Transition in the Cr$_7$Mn Molecular Nanomagnet

A viable qubit must have a long coherence time $T_2$. In molecular nanomagnets $T_2$ is often limited at low temperatures by the presence of dipole and hyperfine interactions, which are often mitigated through sample dilution, chemical engineering and isotope substitution in synthesis. Atomic-clock transitions offer another route to reducing decoherence from environmental fields by reducing the effective susceptibility of the working transition to field fluctuations. The Cr$_7$Mn molecular nanomagnet, a heterometallic ring, features a clock transition at zero field. Both continuous-wave and spin-echo electron-spin resonance experiments on Cr$_7$Mn samples diluted via co-crystallization, show evidence of the effects of the clock transition with a maximum $T_2\sim350$ ns at 1.8 K. We discuss improvements to the experiment that may increase $T_2$ further.

cond-mat.mes-hall↗

arXiv2016

Precision ESR Measurements of Transverse Anisotropy in the Single-molecule Magnet Ni$_4$

We present a method for precisely measuring the tunnel splitting in single-molecule magnets using electron-spin resonance, and use these measurements to precisely and independently determine the underlying transverse anisotropy parameter, given a certain class of transitions. By diluting samples of the SMM Ni$_4$ via co-crystallization in a diamagnetic isostructural analogue we obtain markedly narrower resonance peaks than are observed in undiluted samples. Using custom loop-gap resonators we measure the transitions at several frequencies, allowing a precise determination of the tunnel splitting. Because the transition under investigation occurs at zero field, and arises due to a first-order perturbation from the transverse anisotropy, we can determine the magnitude of this anisotropy independent of any other Hamiltonian parameters. This method can be applied to other SMMs with tunnel splittings arising from first-order transverse anisotropy perturbations.

cond-mat.mes-hall↗

arXiv2016

Observation of tunneling-assisted highly forbidden single-photon transitions in a Ni$_4$ single-molecule magnet

Forbidden transitions between energy levels typically involve violation of selection rules imposed by symmetry and/or conservation laws. A nanomagnet tunneling between up and down states violates angular momentum conservation because of broken rotational symmetry. Here we report observations of highly forbidden transitions between spin states in a Ni$_4$ single-molecule magnet in which a single photon can induce the spin to change by several times $\hbar$, nearly reversing the direction of the spin. These observations are understood as tunneling-assisted transitions that lift the standard $Δm = \pm 1$ selection rule for single-photon transitions. These transitions are observed at low applied fields, where tunneling is dominated by the molecule's intrinsic anisotropy and the field acts as a perturbation. Such transitions can be exploited to create macroscopic superposition states that are not typically accessible through single-photon $Δm = \pm 1$ transitions.

cond-mat.mes-hall↗